DC-Voltage-Ratio Control Strategy for Multilevel Cascaded Converters Fed With a Single DC Source

Recently, a multilevel cascaded converter fed with a single DC source has been presented. An analysis of the steady-state working limits of this type of converter is presented in this paper. Limits of the maximum output voltage and the minimum and maximum loading conditions for stable operation of the converter are addressed. In this paper, a way to achieve any DC voltage ratio (inside the stable operation area of the converter) between the H-bridges of the single-DC-source cascaded H-bridge converter is presented. The proposed DC-voltage-ratio control is based on a time-domain modulation strategy that avoids the use of inappropriate states to achieve the DC-voltage-ratio control. The proposed technique is a feedforward-modulation technique which takes into account the actual DC voltage of each H-bridge of the converter, leading to output waveforms with low distortion. In this way, the dc voltage of the floating H-bridge can be controlled while the output voltage has low distortion independently of the desired DC voltage ratio. Experimental results from a two-cell cascaded converter are presented in order to validate the proposed DC-voltage-ratio control strategy and the introduced concepts.Ministerio de Ciencia y Tecnología TEC2006-03863Junta de Andalucía EXC/2005/TIC-117

Dynamic behaviour of multi-terminal VSC-based HVDC after a converter outage: DC control strategy

The aim of this paper is to evaluate the effect of DC-voltage control strategy on dynamic behaviour of multi-terminal Voltage-Source Converter (VSC)-Based HVDC after a converter outage. In this paper, two dc voltage control strategies are considered: (i) standard voltage margin method (SVMM) and (ii) standard voltage-droop method (SVDM). The impact is evaluated in this paper using time-domain simulations on simple test system using DIgSILENT PowerFactory considering a sudden disconnection of a converter-station. Simulation results demonstrate how important is the dc-voltage control strategy and the location/number of dc-buses involved in the dc-voltage on the dynamic response of the MTDC systems. The voltage margin control is capable to survive a converter outage just if this converter is operating on constant power mode

Passivity-Based Control Design Methodology for UPS Systems

This paper presents a passivity-based control (PBC) design methodology for three-phase voltage source inverters (VSI) for uninterruptable power supply (UPS) systems where reduced harmonic distortions for the nonlinear load, reduced output voltage overshoot, and a restricted settling time are required. The output filter design and modification for efficient control and existing challenges with the assignment of scaling coefficients of the output voltage, load, and inductor currents are addressed and analyzed. Notably, special attention is given to the modulator saturation issue through implementing an accurate converter model. Applications of the two versions of PBC in three-phase voltage source inverters using stationary αβ and rotating dq frames for a constant frequency of the output voltage are presented. Furthermore, the influence of the PBC parameters on the power converter performance is investigated. A comparative simulation and the experimental results validate the effectiveness of the presented passivity-based control design methodology

The Alternate Arm Converter (AAC) - "short-overlap" mode operation - analysis and design parameter selection

This paper presents converter operation principles and theoretical analyses for “short-overlap” mode operation of the Alternate Arm Converter (AAC), which is a type of modular multilevel Voltage Source Converter (VSC) that has been proposed for HVDC transmission applications. Fourier series expressions for the ideal arm current and reference voltage are derived, for the first time, in order to develop an expression for the sub-module capacitance required to give a selected peak-peak voltage ripple of the summed sub-module capacitor voltages in an arm. The DC converter current contains non-negligible low order even harmonics; this is verified by deriving, for the first time, a Fourier series expression for this current. As the DC converter current needs to be filtered to form a smooth DC grid current, a novel DC filter arrangement is proposed, which uses the characteristics of a simplified DC cable model, as well as the capacitance of the DC link and additional DC link damping resistance, in order to form a passive low pass filter. Results obtained from a simulation model, which is based on an industrial HVDC demonstrator, are used in order to verify the presented converter operation principles and theoretical analyses

Multivariable control for a three-phase rectifier based on deadbeat algorithm

This paper presents a deadbeat control technique applied in a voltage source rectifier to regulate both the dc voltage (active power) and reactive power injected to the grid. As the deadbeat control is based on the system model, it leads to a faster response, without overshoot and no need to tune the controller parameters. Hence, it is used to fully control the voltage source rectifier, achieving a fast dynamic response for both the dc voltage and the power factor at the point of connection. However, there are some issues related to the high amount of power required to reach the references –especially in the dc voltage¬ in a few control steps. The proposed technique also protects the equipment by limiting the maximum power drained to/from the source. The mathematical development is made as a function of the converter power in order to limit it, but at the same time tracking the references with high dynamics, characteristic typical of deadbeat control

Unbalanced Variable Nonlinear Load Compensation Using Multiple Shunt Active Filters

The proposed scheme has considered a three-phase four-wire system, which experienced sag and swell in source voltage for a certain period while feeding an unbalanced and variable non-linear load. The load has unequal resistive and reactive elements in the three phases, forming the unbalanced component. A three-phase silicon controlled rectifier converter with adjustable firing angle connected to the load has formed the variable non-linear component. This has been considered, so as to simulate the unbalanced and variable non-linear  nature of loads in real-time power system. The trends in the total harmonic distortion variation were obtained for the proposed system under power factor correction and voltage regulation mode operation when the load-side converter firing angles of 30°, 60° and 90° were considered using MATLAB/SIMULINK software. Three pulse-width-modulation methods, namely, sinusoidal pulse-width-modulation, space vector modulation and hysteresis pulse-width-modulation have been used to generate pulses for the voltage source converter of the shunt active filter based on the reference currents generated using synchronous reference frame theory.  It has been demonstrated in the proposed paper that power factor correction, voltage regulation, better harmonic reduction and hence load compensation are obtained simultaneously by using two SAFs

DC-DC converter based photovoltaic simulator with a double current mode controller

This thesis presents the performance of a DC-DC converter based with current mode controller photovoltaic (PV) simulator which emulates the output characteristics of a real PV module. A portable PV simulator prototype of 85 W is examined in terms of its steady state IV curve matching capacity and the convergence time corresponding to step change in current source load, voltage source load, and insolation levels. The current-voltage (IV) characteristics of the PV module are implemented as a look-up-table (LUT) which determines the reference output current from measured output voltage. A Thevenin Equivalent Method approach to PV arrays analysis is also included in order to model the small-signal linearized characteristics. Extensive simulation results obtained in MATLAB are included to show that the PV simulator can work in most situations as a real PV module. Experimental results verify performance with current source and voltage source loads --Abstract, page iii

A single-stage three-phase DC/AC inverter based on Cuk converter for PV application

This paper presents a new three-phase dc-ac inverter based on the basic Cuk converter. The main feature of the proposed topology is the fact that the energy storage elements as inductors and capacitors values can be reduced in order to improve the reliability, reduce the size, and the total cost. Moreover, the bucking-boosting inherent nature of the Cuk converter, depending on the time-varying duty ratios, provides more flexibility for stand-alone and grid connected applications when the required output AC voltage is lower or greater than the DC side voltage. This property is not found in the conventional current source inverter (CSI) when the DC input current is always greater than the ac output one or in the conventional voltage source inverter (VSI) as the output ac voltage is always lower than the dc input one. Averaged large and small signal models are used to study the Cuk nonlinear operation. Basic structure, control design, and MATLAB/SIMULINK results are presented in this paper. The new three-phase DC-AC inverter is very convenient for PV applications where continuous average input currents are required for appropriate Maximum power Point Tracking (MPPT) operations

Directional Derivative-Based Method for Quasi-Stationary Voltage Support Analysis of Single-Infeed VSC-HVDC units

This study presents an investigation of the impact of the quasi-stationary voltage support provided by a voltage source converter (VSC) connected to a single point of a power system. Based on the directional derivative concept, an analytical method is developed to quantify the sensitivities of the AC bus voltage with respect to the VSC reactive power control modes. Based on a real case study, it is shown that the method applies to VSC units that are part of VSC-HVDC systems, which can operate in a point-to-point or multi-terminal configuration. Time-domain simulations are performed to verify the findings from the application of the analytical method on a reduced size power system.</p